Add support for .Net Core

Presently the thrift compiler C# generator includes the line:

using System.Runtime.Serialization;

And uses the annotation:

#if !SILVERLIGHT
    [Serializable]
#endif

Binary serialization was dropped from .Net Core. As .Net Core is the "go forward" cross platform engine for .Net it would be great if we could improve the C# impl such that it runs on .Net Framework and .Net Core.

Ref:
https://blogs.msdn.microsoft.com/dotnet/2016/02/10/porting-to-net-core/